Scroll-type apparatus is known for operating compressors or vacuum pumps. In both cases the apparatus comprises two scroll members each comprising a flat end plate on which are upstanding strips defining a spiral (or scroll) type structure which interact by placing the scroll members together with the respective spiral type structures inter-engaging and allowing one scroll member to “orbit” relative to the other. In this way it is possible to trap a volume of gas between the scroll members thereby to urge the gas from one end of the respective spirals to the other. In a vacuum pump in particular the gas enters via a pump inlet at the periphery of the scroll members and exits via a pump outlet at the centre of the scroll members.
FIG. 1 illustrates one known arrangement of such scroll-type apparatus in the form of a vacuum pump 2, which includes a stator 4 comprising a fixed scroll member 6. Complimenting the fixed scroll member 6 and intermeshing therewith in a manner known per se is an orbital scroll member 8. The orbital member 8 is mounted on bearings 10, which are supported on a crank end 12 of a drive shaft 14 extending from a motor 16. Bearings 17 serve to support the drive shaft 14 relative to the housing 4. Each scroll member 6, 8 includes an end plate to which is attached and from which extends outwardly therefrom a spiral wrap 18, 20. The height of each wrap 18, 20 is substantially the same as the distance between the opposed end plates so that the free edge of each wrap 18, 20 forms a seal against the face of the complimentary scroll member end plate. To assist in the sealing process, complaint tip seals (not shown) are commonly provided at the free edge of each wrap 18, 20 to take up the axial clearance between the edge of each wrap 18, 20 and the end plate of the opposing scroll member.
In use, a pumped gas is drawn into the pump 2 from inlet 22 and exits via outlet 24 and non-return valve 25. In order to seal the vacuum space in the housing 4 from ambient atmosphere and the pump drive system, a metal bellows 26 is disposed substantially co-axially with the drive shaft 14. The metal bellows is attached at one end by bolts 28 to the orbital scroll member 8 and at its opposite end to a stationary part 30 of the pump by bolts 32. O-rings 29, 31 serve to seal the attachment of the metal bellows 26.
By virtue of the torsional stiffness of the metal bellows 26, the bellows 26 also serves to limit undesirable travel in the circular translation of the orbital scroll member 26 and thus maintain the correct angular position between the fixed scroll member 6 and the orbital scroll member 8. However, the use of such bellows restricts the design of any vacuum pump incorporating such bellows to the dimensions of adequately sized bellows having the required torsional stiffness and sufficient radial freedom for required crank throw to achieve in practice an acceptable life.
Furthermore, the use of compliant tip seals to provide axial sealing between the scroll members 6, 8 can generate the dust due to erosion of tip seals during use of the pump. Whilst this dust is generally conveyed through the outlet 24 together with the exhaust gas, when the pump is switched off this dust could migrate through the inlet 22 towards the evacuated apparatus, leading to unwanted contamination of the evacuated apparatus. The tip seals therefore require regular replacement, and their use further restricts the design of any vacuum pump incorporating such seals to the dimensions of adequately sized tip seals.
It is an aim of at least the preferred embodiment of the present invention to solve these and other problems.